1. Field of the Invention
This invention relates to a soft decision circuit provided in a receiver for receiving a modulated wave modulated by four-phase PSK, and more particularly to a soft decision circuit which codes a received wave at a stage following an intermediate frequency (IF) stage of a receiver using maximum ratio synthetic diversity reception.
2. Description of the Related Art
In radio communications, error correction is performed to assure a sufficient transmission quality. As an error correction code, a BCH code or a convolution code is employed, and among convolution codes, a viterbi decoding method which has a high correction capacity is used frequently.
In a viterbi decoding method, a soft decision circuit which performs soft decision to obtain a higher coding gain is used. In the soft decision circuit, a receive level signal or a determination error upon decoding is used as reliability information (refer to, for example, "Digitization Technique for Mobile Communications", Triceps, 1990, p. 158).
In the conventional soft decision circuit described above, a receive level signal or a determination error upon decoding is employed as reliability information. When a receiver performs diversity reception in order to reduce a bad influence of multi-path fading, rainfall attenuation or the like, if selective diversity reception change over to one of a plurality of receive signals having different characteristics, then a receive level signal or a determination error of a selected system may be employed as reliability information. However, if maximum ratio synthetic diversity reception which has a good noise characteristic is performed, then a receive level signal after synthesized from a plurality of signals must be used as reliability information. To this end, complicated calculations must be performed using software. Accordingly, the conventional soft decision circuit has a problem in that much time is required for the processing and consequently the circuit cannot actually be used as a practical circuit. Further, depending upon a transmission line characteristic, error correction based on hard decision is not always sufficiently effective, and the conventional soft decision circuit has another problem in that, if soft decision is used where the transmission line characteristic is such as just described above, much processing time is required.